The present invention relates to a procedure for the preparation of a novolac type phenolic resin that is quick-curing and has high fluidity. It can be utilized as a binder for various types of organic or inorganic materials.
Novolac type phenolic resins are generally prepared by reacting a phenol and an aldehyde in the presence of an acidic catalyst. Usually, a phenol and an aldehyde are placed in a reaction vessel with an acidic catalyst and reflux for a period of time, followed by dehydration under reduced pressure to produce a resin. Hexamine is a preferred agent for curing the resin, and after mixing and forming, heat is applied to form an insoluble and non-fusible cured resin.
When a comparatively strong acid substance such as oxalic acid, hydrochloric acid, para-toluenesulfonic acid, phosphoric acid or the like is used as a reaction catalyst for a novolac type phenol resin, the novolac type phenol resin obtained has a combination form generally called random novolac or high-para-novolac due to a low ortho/para ratio. Although the durability of this cured resin is excellent, it has disadvantages in that the curing rate by heating is low.
As a method of improving the curing rate of a novolac type phenol resin, there is known a method of using a metal salt of an organic acid such as zinc acetate, magnesium acetate and calcium acetate as a reaction catalyst for the synthesis of a resin, thereby obtaining a resin having a high ortho/para ratio and generally called a high ortho-novolac. Although this method brings about a quick curing property, it has the disadvantage in that since the fluidity of the resin during curing is low, the strength of the cured resin is inferior.
A method of adding an aromatic carboxylic acid such as salicylic acid and benzoic acid to a resin during or after synthesis of a resin has been generally used. This has the effects of accelerating curing, lowering of the melting point and increasing the fluidity of the resin.
Therefore, it is an object of this invention to obtain a novolac type phenolic resin that possesses the rapid cure cycle so desire, and high fluidity for easy incorporation in a formulation.